The present invention relates to band pass filters, and more particularly to such filters adapted for wireless or wired transmitter and receiver applications.
Resonance characteristics of a passive LC serial/parallel band pass filters has been studied thoroughly and are well-known. In a wireless or wired transmitter and receiver, a passive LC band pass filter is often the preferred selective or tuning amplifier unit (single or dual tuning) and is used widely thanks to its low price, operational stability, simple design, ease of frequency changing and superior performance. However, the Q value of the loop will decrease as the operating frequency increases. With increased frequency, the capacitor will consume ohms and distribute inductance in addition to its capacitance characteristic while the inductor will consume ohms and distribute capacitance apart from its inductance characteristic. Such consumption of ohms will increase when the operating frequency increases so that the ohm loss of the loop increases, leading to degradation in the quality of the loop. Due to such imperfections, a passive LC band pass filter is rarely used directly as a tuning amplifier unit at the pre-stage of a high-frequency receiving device with variable frequency for the purpose of improving the signal-to-noise ratio, sensitivity and selectivity of the receiving device. It is because of these shortcomings that wireless receiving devices adopt a superheterodyne mode, i.e., mixing, intermediate frequency amplification, wave detection, low frequency amplification and power amplification and place the majority of the amplification required by the receiving device on the intermediate frequency amplification where frequency is fixed, and the minority of the amplification on the subsequent low frequency amplification.
A passive LC serial band pass filter is often used at the pre-stage of a high frequency receiver with small source resistance. Generally, it is a single-tuning resonance system and requires, when working under high frequency, an inductor that has a high quality factor, i.e. an inductor made of silver-coated hollow conducting wire. Even so, the system will still have a relative high shape factor. A passive LC parallel band pass filter is widely used in low frequency signal transmitting and detecting devices with relatively low working frequency and large source resistance in radio engineering, acoustical engineering and mechanical vibration applications.
There have been many studies on active RC high pass filters, low pass filters and band pass filters made of passive R and C elements as well as operational amplifiers by using Fourier transformations and from the perspective of the frequency domain of a network. There also have been many studies on band pass filters made of high and low pass filters. However, change of resonance frequency of these band pass filters requires simultaneous, synchronous and precise change of the value of the parameters of several R and C elements, making change of frequency extremely complicated and difficult, particularly for high-performance band pass filters using multiple dual tuning units connected in series. Therefore, this type of active RC band pass filters is almost limited to use in receiving systems of fixed frequency only. Few studies, however, have been made on an active LC resonance and related theories and patents are rarely seen. Data show that up to date, there is no patent on an active LC band pass filter.
Typically, a receiving system for variable frequencies uses a heterodyne receiver. Currently in a heterodyne wireless receiving system, the high-frequency gain is relatively small. In order to meet the gain requirements for the whole system, the prevailing practice is to put the gain task on the intermediate frequency amplifier which can work stably at fixed frequency. In order to improve the sensitivity of the whole system, common practice is to rely on a large antenna or by using an additional antenna amplifier, or in a technically complicated scenario, using a dual tuning band pass filter made of two electric tuning diodes with variable capacitance at the pre-stage of the input to amplify the high frequency component at small powers.
Thus there is a need, particularly in the field of both wired and wireless receiving systems, for a high-performance input unit that is capable of high voltage gain, good selectivity, powerful noise suppression, high SNR, ease of input/output matching and stable operation to replace the existing passive LC band pass filter. In addition, the replacement should be very simple and can be made very easily. The input unit should work very stably.